This invention pertains to apparatus and methods for medical ventilation and, more particularly, to apparatus and methods for controlling the pressure of gases within a patient's mouth or respiratory tract during medical ventilation.
There are a number of ventilatory techniques or ventilatory modes used by physicians to assist a patient's respiratory function and to reabilitate his or her respiratory capacity. These techniques or modes include positive end-expiratory pressure (PEEP), intermittent mandatory ventilation (IMV), continuous positive airway pressure (CPAP), intermittent positive pressure breathing (IPPB), high-frequency ventilation (HFV) and others. Since the optimum pressure, flow and gas-volume characteristics differ for each of these techniques or modes and, for any given mode, from patient to patient, a medical ventilator designed to perform optimally in one of these modes and for a particular type of patient is unlikely to perform optimally in other modes and for other types of patients. A medical ventilator capable of continuous, high speed control of the pressure of gases within a patient's mouth in accordance with any selected pressure waveform and for any patient's respiratory characteristics, however, could optimally perform all of these ventilatory modes with a minimum of patient-effort and with a high degree of patient-comfort. Such a ventilator also could enable the development of entirely new ventilatory techniques. No such medical ventilator presently exists.
U.S. Pat. No. 4,838,257 to Hatch discloses a medical ventilator with pressure and flow monitors located at various points within the pneumatic circuit for providing feedback to a computer-controller. Hatch states that his system enables one to "select the desired frequency, volume, pressure, phase relationships and waveform of gases being used to ventilate the lungs of a patient." column 6, lines 11-14. Other than showing the use of a computer-controller and the provision of feedback signals, however, Hatch provides little teaching on how to accomplish these goals.
U.S. Pat. No. 4,448,192 to Stawitcke et al. discloses a medical ventilator intended to follow a selected pressure-volume waveform. A controller compares pressure and gas-volume feedback signals, provided from piston-driven pumps, with a desired pressure-volume waveform and generates a pressure-error signal for controlling the pumps. Although Stawitcke et al. also shows the use of feedback to a controller, this reference also provides little teaching on how to control the actual pressure of gas within a pneumatic circuit, particularly the actual pressure within, or closely adjacent to, a patient's mouth or respiratory tract, in accordance with any selected pressure waveform.
U.S. Pat. No. 4,393,869 to Boyarsky et al. discloses a medical ventilator with an electrical controller and an electrical-to-pneumatic transducer "for controlling the pressure of . . . regulated air in accordance with a predetermined electrical signal." column 8, lines 2-4. The Boyarsky et al. feedback system is not a closed loop, however, and, therefore, does not attempt to cause the actual pressure within the pneumatic circuit to follow a pressure represented by the electrical signal or any other desired pressure. Also, although Boyarsky et al. discloses a pressure detector for providing feedback signals to the controller, these signals are used only to monitor the "duration of inspiration and expiration" and to insure closure of an expiratory valve at a time when "a residual pressure [remains] in the lungs," not to provide closed-loop control. column 9, lines 30-31, 34.